Process for operating a yellow flame burner

ABSTRACT

The invention relates to a process for operating a yellow flame burner, wherein the fuel comprises a Fischer-Tropsch product which contains more that 80 wt % of iso and normal paraffins, less than 1 wt % aromatics, less than 5 ppm sulfur and less than 1 ppm nitrogen and wherein the density of the Fischer-Tropsch product is between 0.65 g/cm 3  and 0.8 g/cm 3  at 15 ° C.

The invention is directed to an improved use of a yellow flame burner.

Conventional designs of oil burner assemblies for home heating fuel oils employ a traditional fuel/air mixing process in which the evaporation and combustion of the fuel oil take place simultaneously. In one form of oil burner assembly for home heating fuel oils the fuel oil is sprayed as a hollow cone and air is weakly swirled along a path which is parallel to the axis of a burner blast tube and which passes into the hollow cone so that the trajectories of the fuel oil droplets cross the air flow streamlines. This leads to a rapid evaporation giving fuel oil rich regions, which in turn ignite under local sub-stoichiometric conditions producing soot, and results in air pollution as well as well as a waste of a fossil fuel.

The general pattern of the flame of such an oil burner assembly is one of heterogeneity in terms of fuel concentrations; the pockets of fuel lean mixture give rise to high nitric oxide concentrations from both the fuel nitrogen and the atmospheric nitrogen, while the pockets of fuel rich mixture give rise to soot. The visible flame when using an Industrial Gas Oil fuel from such a system is yellow. The yellow colour is the visible radiation from the high temperature soot particles and this completely masks other visible radiations as far as the human eye is concerned. These soot particles result from non-burnt carbon.

For complete combustion of the carbon, that is soot-free combustion, the step-wise combustion of carbon to carbon dioxide via the intermediate carbon monoxide stage gives rise to a visible radiation in the blue region of the light spectrum. When this occurs the blue radiation becomes visible in a soot-free or low-luminosity flame, and oil burners for such soot-free flames are known as blue flame burners.

Blue flame burners are known to have a desirable low NOx emission as compared to yellow flame burners. Nevertheless yellow flame burners are still widely applied and there is thus a need to reduce the NOx of such yellow flame burners.

This object is achieved by the following use. Use of a Fischer-Tropsch derived fuel in a yellow flame burner.

Applicants have found that the low NOx emissions of a yellow flame burner can be reduced when a Fischer-Tropsch derived fuel is used. Applicants have even found that the NOx emission can be reduced to below the level of a blue flame burner using conventional Industrial Gas Oil as fuel. An even further advantage is that the carbon monoxide emission is reduced. A next advantage is that less odour during start and extinction of the yellow flame burner has been observed when using this fuel. This is very advantageous, especially when such a burner is used in a domestic environment, wherein frequent start and stops of the burner are common. A next advantage is that the carbon monoxide and hydrocarbon emissions at the cold or hot start of the yellow flame burner are less as compared to when state of the art oil is used. This is also very advantageous when the burner is used in for example a domestic heating application wherein frequently the burner has to start and stop.

FIG. 1 shows a schematic representation of such a yellow flame burner.

FIG. 1 shows a yellow flame burner 1 having pumping means 2 to supply a liquid fuel and a van 3 to supply an oxygen containing gas. The oxygen containing gas is usually air. The fuel is dispersed in a nozzle 4 and mixed with the air to form a combustible mixture, which is fed to a combustion space 5 via a conical shaped nozzle 6. FIG. 1 also shows means 7 to ignite the mixture.

The operating conditions of the yellow flame burner may be the same as the operating conditions used for the state of the art fuels. The proportion of air in excess of that required for stoichiometric combustion is known as the excess air ratio or “lambda”, which is defined as the ratio of total air available for combustion to that required to burn all of the fuel. Preferably the lambda is between 1 and 2 and more preferably between 1 and 1.6. Applicants found that by using a Fischer-Tropsch derived fuel a very low lambda of between 1.05 and 1.2 could be applied without large emissions of carbon monoxide as would be the case when Industrial Gas Oil would be used.

The yellow flame burner using the Fischer-Tropsch fuels is preferably applied for domestic heating, wherein the heat of combustion is used to heat water by indirect heat exchange in so-called boilers. The heated water may be used to warm up the house or consumed in for example showers and the like. More preferably the yellow-flame burner is used in (domestic) application wherein more than 3 starts of the burner per hour takes place. The use of the present invention is especially suited for such applications because low hydrocarbon and carbon monoxide emissions have been found at the start of the burner running on the Fischer-Tropsch derived fuel.

The yellow flame burner using the Fischer-Tropsch fuels may advantageously be further used for direct heating of large spaces. Such applications are characterized in that the flue gasses are directly supplied to said space to heat up said space. Spaces such as tents and halls are often heated up with such an apparatus. Normally gaseous fuels for example natural gas, LPG and the like, are used for this application because the associated flue gasses can be safely supplied to said space. A disadvantage of the use of gaseous fuels is however that handling of the pressurized gas containers and combustion equipment requires professional skills in order to operate such an apparatus safely. By using a Fischer-Tropsch derived liquid fuel a comparable flue gas is obtained in the yellow flame burner as when a gaseous fuel is used. Thus a method is provided wherein a liquid fuel can be applied for direct heating of spaces. The application of the liquid Fischer-Tropsch derived fuel makes the use of the apparatus for direct heating much more simple and safe.

The Fischer-Tropsch derived fuel will comprise a Fischer-Tropsch product which may be any fraction of the middle distillate fuel range, which can be isolated from the (hydrocracked) Fischer-Tropsch synthesis product. Typical fractions will boil in the naphtha, kerosene or gas oil range. Preferably a Fischer-Tropsch product in the kerosene or gas oil range is used because these fractions are easier to handle in for example domestic environments. Such products will suitably comprise a fraction larger than 90 wt % which boils between 160 and 400° C., preferably to about 370° C. Examples of Fischer-Tropsch derived kerosene and gas oils are described in EP-A-583836, WO-A-9714768, WO-A-9714769, WO-A-011116, WO-A-011117, WO-A-0183406, WO-A-0183648, WO-A-0183647, WO-A-0183641, WO-A-0020535, WO-A-0020534, EP-A-1101813, U.S. Pat. No. 5,766,274, U.S. Pat. No. 5,378,348, U.S. Pat. No. 5,888,376 and U.S. Pat. No. 6,204,426.

The Fischer-Tropsch derived product will suitably contain more than 80 wt %, preferably more than 90 wt % iso and normal paraffins and less than 1 wt % aromatics, the balance being naphthenics compounds. The content of sulphur and nitrogen will be very low and normally below the detection limits for such compounds. This low content of these elements is due to the specific process wherein the Fischer-Tropsch reaction is performed. The content of sulphur will therefore be below 5 ppm and the content of nitrogen will be below 1 ppm. As a result of the low contents of aromatics and naphthenics compounds the density of the Fischer-Tropsch product will be lower than the conventional mineral derived fuels. The density will be between 0.65 and 0.8 g/cm³ at 15 C.

The fuel used in the process of the present invention may also comprise fuel fractions other than the Fischer-Tropsch derived fuel components. Examples of such components may be the kerosene or gas oil fractions as obtained in traditional refinery processes, which upgrade crude petroleum feedstock to useful products. Preferred non-Fischer-Tropsch fuel fractions are the ultra low sulphur (e.g. less than 50 ppm sulphur) kerosene or diesel fractions, which are currently on the market. Optionally non-mineral oil based fuels, such as bio-fuels, may also be present in the fuel composition. The content of the Fischer-Tropsch derived product in the fuel will be preferably be above 40 wt %, more preferably above 60 wt % and most preferably above 80 wt %. It should be understood that the content of such, currently less available, Fischer-Tropsch products will be optimised, wherein pricing of the total fuel will be balanced with the advantages of the present invention. For some applications fuels fully based on a Fischer-Tropsch product plus optionally some additives may be advantageously used.

Yellow flame burners are often provided with a flame detector. Most detectors, which are used today, detect a particular wavelength associated with the yellow colour of the flame. Applicants have now found that when a Fischer-Tropsch derived fuel is used the commonly known detectors fail to observe the resulting blue coloured flame. For this reason the yellow flame burner is preferably provided with a detector, which can detect this blue flame. Examples of suitable detectors are the detectors that are used in blue flame burners. Examples of suitable detectors are the UV sensors and IR sensors. A more preferred detector is the so-called ionisation sensor. An ionisation sensor is suitable to monitor burners with intermittent operation as well as continuous operation. The principle of operation of the ionisation flame monitor is based on the rectifying effect of a flame. If a flame is present, a current flows between the burner an the ionisation electrode. This ionisation current is evaluated by the flame monitor to determine if a flame is present. In some prior art applications ionisation sensors could not be used in combination with a liquid fuel because deposits in the sensor led to false currents in the sensor. Because use of the Fischer-Tropsch derived fuel, especially a fuel composition not containing a metal based combustion improver additive, results in less deposits ionisation sensors can be applied. This is an advantage because these sensors are more readily available than the IR or UV sensors.

Alternatively additives may be added to the Fischer-Tropsch derived fuel which result in a flame which can be detected by the above standard yellow flame burner detector. Examples of possible additives are azo dyes and alkali metal based additives, for example based on Na or K.

The fuel may also comprise one or more of the following additives. Detergents, for example OMA 350 as obtained from Octel OY; stabilizers, for example Keropon ES 3500 as obtained from BASF Aktiengesellchaft, FOA 528A as obtained from OCTEL OY; metal-deactivators, for example IRGAMET 30 (as obtained from Speciality Chemicals Inc; (ashless) dispersants, for example as included in the FOA 528 A package as obtained from Octel OY; anti-oxidants; IRGANOX L57 as obtained from Specialtiy Chemicals Inc; cold flow improvers, for example Keroflux 3283 as obtained from BASF Aktiengesellschaft, R433 or R474 as obtained from Infineum UK Ltd; anti-corrosion: Additin RC 4801 as obtained from Rhein Chemie GmbH, Kerocorr 3232 as obtained from BASF, SARKOSYL 0 as obtained from Ciba; re-odorants, for example Compensol as obtained from Haarman & Reiner; biocides, for example GROTA MAR 71 as obtained from Schuelke & Mayr; lubricity enhancers, for example OLI 9000 as obtained from Octel; dehazers, for example T-9318 from Petrolite; antistatic agents, for example Stadis 450 from Octel; and foam reducers, for example TEGO 2079 from Goldschmidt.

Applicants found that metal-based combustion improvers, which typically are added to the fuel composition used in the prior art method, can be left out of the fuel. This is advantageous because as explained above ionisation sensors may then be advantageously applied. Metal based combustion improvers are for example ferrocene, methylcyclopentadienylmanganese-tricarbonyl (MMT).

The Fischer-Tropsch derived product is colourless and odourless. For safety reasons an odour marker, as for example applied in natural gas for domestic consumption, may be present in the Fischer-Tropsch derived fuel. Also a colour marker may be present to distinguish the fuel from other non-Fischer-Tropsch derived fuels.

The total content of the additives may be suitably between 0 and 1 wt % and preferably below 0.5 wt %.

The invention will now be illustrated with the following non-limiting examples.

EXAMPLE 1

To a yellow flame burner of Type 800 ULV-S (Shell Direct GmbH) as placed in a PKR-140 boiler (Oertli Rohleder Waermetechnik GmbH) a Fischer-Tropsch derived kerosene (Oil A), a Fischer-Tropsch gas oil (Oil B), an ultra low sulphur gas oil (Oil D) and a standard industrial gas oil (Oil C) having the properties as listed in Table 1 was fed at different lambda. The oils contained the same standard additive package. TABLE 1 Fischer- Fischer- Tropsch Tropsch kerosene gas oil Reference Reference (A) (B) oil-1 (C) oil-2 (D) Density (at 734.8 785.2 854.3 846.3 15° C. in kg/m³ Sulphur content <0.0005 <0.0005 0.142 0.061 (wt %) Kinematic 1.246 6.444 3.842 4.621 viscosity at 20° C. (mm²/s) Flash point 43 92 64 66 (° C.)

During the experiment the NOx content was measured by chemoluminescence. In FIG. 2 the NOx emission relative to the energy input is shown at different Lambda values for the fuels A-D. The energy in kWh is calculated from the amount of fuel fed to the burner and its caloric value. It is clear that the NOx emissions are lower for the Fischer-Tropsch derived fuels as compared to when a normal gas oil or an ultra low sulphur gas oil is used.

The carbon monoxide emission was also measured. In FIG. 3 the CO emission relative to the energy is presented for different values of lambda for oils A-D.

EXAMPLE 2

To a blue flame burner of type Gulliver BLU BGI (Riello) as placed in a Vitola 200 Boiler (Viessmann Werke Gmbh&Co) a conventional gas oil D was supplied. The NOx emissions and power were measured at different lambda. The NOx values were found to be larger than 140 mg/kWh. These values are higher than when using a yellow flame burner and the Fischer-Tropsch derived fuel as illustrated in FIG. 2.

EXAMPLE 3

Example 1 was repeated for oils A, B and D. The hydrocarbon and carbon monoxide emissions were measured at a warm start up. With a warm start up is here meant that the boiler temperature was kept constant at its operating temperature. In FIGS. 4 and 5 the carbon monoxide and hydrocarbon emissions are shown as a function of time. It can be observed that both the CO and hydrocarbon emissions are less when a Fischer-Tropsch derived fuel is used when compared to when conventional gas oil is used.

EXAMPLE 4

Example 1 with the Fischer kerosene was repeated using a flame detector. The flame detector was a so-called photo-element, which delivers amperes (mA) as an output signal. A high output signal is desirable to make correct detection of a flame possible. The output signal of the neat Fischer Tropsch kerosene was 52.7 mA.

In order to increase the output signal to a higher level 0.1 wt % of a cyclo-hexane butanoic acid sodium salt was added. The output signal when the additivated fuel was used was 57.4 mA. Such higher signal outputs are beneficiary to make the flame sensor system less sensitive for system fluctuations. 

1. A process for operating a yellow flame burner comprising: burning a Fischer-Tropsch derived fuel in the burner to obtain flue gases and a flame.
 2. The process of claim 1, wherein lambda is between 1 and 1.6
 3. The process of claim 2, wherein lambda is between 1.05 and 1.2.
 4. The process of claims 1, further comprising heating water by means of indirect heat exchange with the flue gasses in a boiler.
 5. The process of claims 1, further comprising heating a space directly with the flue passes.
 6. The process of claim 1, wherein the Fischer-Tropsch derived fuel boils for more than 90 wt % between 160° C. and 400° C.
 7. The process of claim 6, wherein the Fischer-Tropsch derived fuel boils for more than 90 wt % between 160° C. and 370° C.
 8. The process of claims 1, wherein the Fischer-Tropsch derived fuel comprises a Fischer-Tropsch product which contains more than 80 wt % of iso and normal paraffins, less than 1 wt % aromatics, less than 5 ppm sulfur and less than 1 ppm nitrogen and wherein the density of the Fischer-Tropsch derived product is between 0.65 and 0.8 g/cm³ at 15° C.
 9. The process claim 1, wherein the Fischer-Tropsch derived fuel comprises more than 80 wt % of a Fischer-Tropsch product.
 10. The process of claim 9, wherein the Fischer-Tropsch derived fuel comprises a mineral oil fraction and/or a non-mineral oil fraction.
 11. The process claims 1, further comprising starting the yellow flame burner more than three times per hour.
 12. The process of claim 1, wherein the Fischer-Tropsch derived fuel comprises one or more additives.
 13. The process of claim 12, wherein the Fischer-Tropsch derived fuel comprises an odor marker.
 14. The process of claim 12, wherein the Fischer-Tropsch derived fuel comprises a color marker.
 15. The process claims 12, wherein an additive is present which changes the color of the flame such that is detectable by a yellow flame detector.
 16. The process of claim 1, wherein a blue flame detector is used to detect the yellow flame burner flame.
 17. The process of claim 16, further comprising detecting the flame of the yellow flame burner with an ionization type flame detector and wherein the fuel does not contain a metal-based combustion improver. 